Switch back to the layers dialog. Click the text layer to make it active. Make sure the text layer is exactly the same size as the whole image (Layer / Layer to Image size! [it should already be, because we did that in step 2 – it’s just to make sure ;)]).

This looks like a wonderful tutorial that I would like to apply to numerous sigpics. I am using v.2.6.8, however, I cannot get past Step 3 as there is no option to activate “Initialize from selection” in the channels dialog. If I right Click the 'Depths' channel I have the following options: Channel to Selection, Add to Selection, Subtract from Selection and Intersect with Selection.
What am I missing?